The present invention generally relates to communication systems and signals, and particularly relates to communication channel estimation.
Reliable data reception depends on accurate channel estimation. For example, channel estimates may be used to compensate unknown received data symbols for expected channel distortion, thereby improving symbol demodulation. Channel estimates also play a key role in channel quality estimation, which is an increasingly important aspect of rate and/or power control in evolving wireless communication systems, such as Wideband Code Division Multiple Access (WCDMA) systems.
As a general proposition, one may assume that the propagation channels are frequency selective, meaning that channel estimation for a given propagation channel is frequency dependent. Frequency dependency complicates channel estimation where it is useful to have channel estimates for multiple frequencies of interest. For example, Orthogonal Frequency Division Multiplexing (OFDM) transmission uses a plurality of sub carriers distributed across a given frequency range, where the sub carriers are orthogonal to one another. OFDM finds use in a variety of systems, such as the 802.11a/g standards for Wireless Local Area Networks (WLANs). Evolving wide area communication standards, such as the Universal Mobile Terrestrial Systems (UMTS) standards, also contemplate the use of OFDM.
Several aspects of OFDM make it particularly advantageous for wireless transmissions in multiple access systems. For example, the sub carrier data rate can be varied to suit frequency-dependent channel conditions that vary across sub carriers. Further, different subsets of different sub carriers may be used to serve different users, allowing concurrent transmissions to/from potentially many different users in a wireless communication system.
Thus use of OFDM presents several challenges in terms of channel estimation, however. For example, the particular sub carriers assigned to a particular user may not represent the optimal assignment for that user—i.e., other ones of the sub carriers may offer better performance given the frequency-dependent nature of channel conditions. However, optimizing sub carrier assignments for users is not possible without having channel estimates available for the non-assigned frequencies.